EP0986653B1 - Sintered mechanical part with abrasionproof surface and method for producing same - Google Patents

Abstract

The invention concerns a mechanical part with abrasionproof surface characterized in that it comprises a sintered metallic body obtained from metallic powders and a laser-deposited cermet coating. The coating has a certain thickness whereof a portion is metallurgically bound with the metallic body. The laser deposit enables the sintered part to be surface-melted under the effect of the laser beam. The surface of the sintered part to be coated is therefore fused over a thickness ranging between 10 mum and 1 mm, which enables the surface pores to be closed, as is characteristic of sintered parts, thereby increasing its resistance to shocks. Moreover, the small surface coated at a given moment by the laser enables the self-hardening of the exposed part, following the beam displacement, by the heat-sink effect of the surrounding metallic volume. The resulting coating also has very low porosity owing to the complete fusion of the powders by laser.

Description

FIELD OF THE INVENTION

The present invention relates to antiabrasion laser surface treatment of a mechanical part. More particularly, the present invention relates to surface treatment of a sintered mechanical part obtained by metallurgy of powders by laser deposition of a cermet coating, the cermet being a material composite formed of ceramic products coated in a metallic binder. The The present invention also relates to a method of manufacturing such a part. mechanical.

DESCRIPTION OF THE PRIOR ART

Coatings composed of spherical tungsten carbides in a nickel-chromium matrix and deposited by laser on cast irons or traditional steel and therefore, not sintered, already exist in the prior art. An example of this type of coating is described by way of example in Canadian patent application No. 2,126,517. Laser deposition is a coating technique that deposits thick layers of very hard material on the surface of a metal part. A continuous CO ₂ laser delivers an infrared beam whose energy is used to surface melt the base metal to be coated as well as the filler metal supplied in the form of a fine powder. A coaxial nozzle traversed in its center by a laser beam allows the arrival and injection of the powders forming the coating, the latter resembling a weld bead. To date, this type of laser deposition has only been used to coat traditional unsintered metal parts, used more particularly in very abrasive conditions.

It is well known in the prior art that the mechanical parts produced by powder metallurgy do not have the physical characteristics to work in tension, abrasion or friction and this is due to the presence of a large number of pores on the surface of these sintered parts, thereby reducing the period of initiation of cracks compared to a forged or machined part. Thus, the surface porosity of parts manufactured by powder metallurgy prevents the production of mechanical parts which must resist shock and / or abrasive wear due to the shortness of the crack initiation period.

SUMMARY DESCRIPTION OF THE INVENTION

An objective of the present invention is to propose a part sintered mechanics obtained by powder metallurgy and having a very high impact, abrasion and friction resistance, as well as very good mechanical resistance of the part body.

un corps métallique fritté obtenu par métallurgie des poudres; et

un revêtement en cermet recouvrant le corps métallique et ayant une surface externe constituant la surface antiabrasion, la pièce mécanique étant caractérisée en ce que:

ledit revêtement est obtenu par dépôt laser en injectant de manière coaxiale dans un faisceau laser un flux d'un mélange de poudres métalliques et de poudres céramiques contenant des carbures de forme sphéroïdale, ledit mélange étant destiné à former ledit revêtement, lequel est caractérisé en ce qu'il est exempt de porosité, est métallurgiquement lié au corps métallique, a une épaisseur de 10 µ-mètres à 1 mm et comprend des carbures de forme sphéroïdale dans une matrice métallique.

More particularly, the present invention relates to a mechanical part with an abrasion-resistant surface, characterized in that it comprises:

a sintered metal body obtained by powder metallurgy; and

a cermet coating covering the metallic body and having an external surface constituting the abrasion-resistant surface, the mechanical part being characterized in that:

said coating is obtained by laser deposition by coaxially injecting into a laser beam a flow of a mixture of metallic powders and ceramic powders containing carbides of spheroidal shape, said mixture being intended to form said coating, which is characterized in that that it is free of porosity, is metallurgically linked to the metallic body, has a thickness of 10 μm to 1 mm and comprises carbides of spheroidal shape in a metallic matrix.

Those skilled in the art will understand that by "metallurgically linked to the body metallic ", means that the coating is fused to the surface of the part sintered, the microstructure at the base of the coating being intimately united to the microstructure of the body of the part.

The mechanical part can include any part traditionally used in very abrasive or high tension conditions, for example, the bark pellets mounted on the debarker arms.

a) prévoir une pièce mécanique frittée obtenue par métallurgie des poudres; et

b) déposer par un procédé laser un revêtement en cermet sur une surface externe de ladite pièce mécanique.

The present invention also relates to a method for manufacturing the mechanical part described above. More particularly, the process is characterized in that it comprises the following stages:

a) provide a sintered mechanical part obtained by powder metallurgy; and

b) depositing by a laser process a cermet coating on an external surface of said mechanical part.

• diriger un faisceau laser sur la surface externe de la pièce, le faisceau laser dégageant une certaine température et fusionnant une certaine épaisseur de ladite surface externe;
• injecter dans le faisceau laser un flux constant d'un mélange de poudres céramiques et de poudres métalliques destinées à former le revêtement cermet, les poudres céramiques ayant une température de fusion plus élevée que la température du faisceau laser et les poudres métalliques ayant une température de fusion moins élevée que la température du faisceau laser, de sorte que le laser fusionne les poudres métalliques du mélange de poudres qui se dépose sur la surface externe de la pièce; et
• déplacer le faisceau laser relativement à la pièce mécanique pour ainsi balayer la surface externe et former le revêtement de cermet.

The laser deposition process preferably includes the following steps:

• directing a laser beam on the external surface of the part, the laser beam releasing a certain temperature and fusing a certain thickness of said external surface;
• injecting into the laser beam a constant flow of a mixture of ceramic powders and metal powders intended to form the cermet coating, the ceramic powders having a higher melting temperature than the temperature of the laser beam and the metal powders having a temperature of melting lower than the temperature of the laser beam, so that the laser fuses the metal powders of the powder mixture which is deposited on the external surface of the part; and
• moving the laser beam relative to the mechanical part thereby sweeping the external surface and forming the cermet coating.

The powder mixture can be injected into the laser beam using a coaxial nozzle traversed in its center by the laser beam, the nozzle allowing the arrival of the powder mixture and its injection into the laser beam.

The laser beam is preferably fixed and the mechanical part is installed on a movable table which can be moved relative to said laser beam.

This coating according to the present invention being deposited by laser makes it possible to melt the sintered part to be coated on the surface under the effect of the laser beam. The surface of the sintered part to be covered is therefore fused over a thickness which can range from 10 μm to 1 mm, which allows the closing of the pores on the surface, typical of sintered parts and, consequently, the increase in its resistance to shock. In addition, the small area covered at a given time by the laser allows the self-soaking of the exposed area, following the movement of the beam, by heat sink effect of the surrounding metal volume. The coating obtained according to the present invention also has a very low porosity due to the complete melting of the metal powders by the laser.

Other objects, characteristics and advantages of the present invention will emerge from the following description of a preferred embodiment, made in relationship with the attached figures.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a perspective view of a debarking arm on which is mounted a sintered debarking disc having an abrasion-resistant coating according to a preferred embodiment of the present invention;

Figure 2 shows schematically in cross section a portion the working surface of the debarking disc of Figure 1;

Figure 3 shows schematically and partly a device for laser reloading for the implementation of the present invention;

Figure 4 is a scanning electron microscope photograph (SEM) showing the microstructure of the joint formed between a coating obtained by plasma spraying on a base metal; and

Figure 5 is a scanning electron microscope photograph (SEM) showing the microstructure of the interface between a coating obtained by laser deposition and the surface of a part produced by powder metallurgy and forming the subject of the present invention.

DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

• dans l'industrie minière: les broyeurs, boulets, concasseurs, convoyeurs, etc.;
• dans l'industrie céramique et connexe: les racleurs, couteaux, moules, vis de convoyeurs, sièges de vannes, etc.;
• dans l'industrie papetière: les plaques de raffineurs, plaques de fonds de pulpeurs, palettes, etc.;
• dans l'industrie métallurgique: les cylindres, bagues, galets, etc.;
• dans l'industrie de la plasturgie: les sommets de filets de vis d'extrusion et d'injection; et
• dans l'industrie agro-alimentaire: les rouleaux, filières, déflecteurs, vis.

FIG. 1 shows a debarking arm (2) for a debarker with a rotating ring, on which arm is mounted a debarking disc (4) produced according to the present invention. This arm (2) comprises a first end (6) adapted to be fixed to the rotating ring of the debarker. The arm (2) comprises a second end (8) constituting the working surface of the arm (2) which is used to remove the bark from a tree as the latter moves longitudinally inside the ring . The patch (4) is operatively attached to this second end. This second end (8) is the part of the arm which is used to bark the trees and must be able to withstand very abrasive conditions. A debarking tablet according to the present invention can therefore advantageously be used, the latter having a very hard cermet coating which can withstand such working conditions. It should be understood that, although the preferred embodiment illustrated here represents a debarker tablet, it is only one example of a mechanical part according to the present invention among many others. Indeed, any mechanical part traditionally used under very abrasive conditions or high tension can be manufactured according to the present invention. The following mechanical parts are other examples of parts which can be manufactured according to the present invention:

• in the mining industry: grinders, balls, crushers, conveyors, etc .;
• in the ceramic and related industry: scrapers, knives, molds, conveyor screws, valve seats, etc .;
• in the paper industry: refiner plates, pulper base plates, pallets, etc .;
• in the metallurgical industry: cylinders, rings, rollers, etc .;
• in the plastics industry: the tops of threads for extrusion and injection screws; and
• in the food industry: rollers, dies, deflectors, screws.

As illustrated in FIG. 2, the debarker tablet (4) with an abrasion-resistant surface, or any other mechanical part according to the present invention, comprises a sintered metal body (10) obtained by powder metallurgy and a coating (12) in cermet covering the metallic body (10). The external surface (14) of the coating constitutes the abrasion-resistant surface of the part. The coating (12) has a certain thickness, a portion of which is metallurgically linked to the metallic body (10), as can be seen in FIG. 5. This portion preferably has from 10 μm to 1 mm.

The coating (12) in cermet is preferably based on carbides of tungsten (16), of titanium carbides or boron carbides, of spheroidal shape in a metal matrix (18).

The metal matrix (18) is preferably based on at least one of the metals selected from the group consisting of nickel, chromium and cobalt, more particularly it includes nickel, chromium and cobalt. Advantageously, the Ni-9% Cr-Co is used.

The coating (12) preferably comprises 65% by weight of carbides of tungsten (16) and is substantially free of porosities.

The coating (12) for sintered part according to the present invention is obtained by laser deposition.

As illustrated in FIG. 3, a coaxial huse (20), which is mounted at the output of a CO ₂ laser beam (22) of 6 kW, injects into the laser beam (22) a constant flow of powders ( 24) of the material to be deposited. The laser beam (22) fuses the powders (24) and welds them to the base metal (4) in the form of a cord constituting the coating (12). By sweeping the surface of the part (4), a coating is formed at the desired locations. The laser coating (12) is composed of tungsten carbide particles (16) of very high hardness in a chromium-nickel matrix (18) and it has excellent resistance to abrasion and erosion wear, as well as very good resistance to corrosion. Figure 4 shows the microstructure of a coating (26) comprising carbides (28) obtained by plasma spraying while Figure 5 shows the microstructure of a laser coating (12) on a sintered part (4). As can be seen, the particles of tungsten carbide (16) found in the coating (12) by laser deposition are spheroidal, while the carbides (28) obtained by the coating (26) plasma projection tend rather to be angular in shape. It is also noted that there has been fusion of the surface of the sintered part (4) with the metal part (18) of the coating (12). This fusion made it possible to close the pores present on the surface of the sintered metal (4).

The laser (22) being fixed, a table (30) with numerical control with four axes on which rest the parts (4) to be coated allows for precise deposits and uniform by relative displacements of the parts (4) relative to the laser beam (22). Coatings with a thickness of less than 1 mm or more than 10 µm, for successive passages of the laser (22) can be carried out.

Materials used in the manufacture of coatings by laser deposition are generally mixtures of powders of tungsten carbide, carbide high purity and very high hardness titanium or boron carbide alloys, depending on the application, metallic powders based on nickel, chromium or cobalt. During the deposition process, the metal powders are fused by the laser (22) while the tungsten carbide powders remain solid, thus preserving their very high hardness. These cermet-type materials give coatings (12) excellent resistance to abrasion and erosion wear, as well very good resistance to corrosion.

Several characteristics of the laser deposit ensure that the coatings (12) produced by this technique have exceptional properties. All first, the deposits produced by laser are metallurgically linked to the base metal (10) and are perfectly dense (absence of porosity). The adhesion obtained between the part (10) and the coating (12) is therefore excellent. In contrast, coatings produced by hot spraying have a high porosity and a special preparation of the treated surfaces to ensure good adhesion.

Very precise control of the energy supply on the base metal allows to obtain dilutions of the base metal in the deposit of less than 1% and to minimize or even eliminate any deformation. In addition, laser deposition allows produce fine metallurgical microstructures thanks to the speed of cooling during treatment, thereby increasing the hardness of the metal matrix (16) (2400 to 3600 HV). Finally, the use of programs and CNC controllers leads to perfectly reproducible deposits in the time and whose final thickness is perfectly controlled. Large series of parts can thus be processed.

INDUSTRIAL APPLICATIONS OF THIS INVENTION

• présence d'un grand nombre de pores en surface;
• faible résistance aux chocs;
• capacité mécanique généralement moindre qu'une pièce forgée;
• plus faible densité;
• absorption du bruit;
• possibilité d'usage d'alliages non miscibles par voie liquide;
• possibilité d'usage d'alliages auto-trempants;
• faible coût de production pour une série de pièces.

A mechanical part manufactured by powder metallurgy but not comprising a coating according to the present invention has the following physical and economic characteristics:

• presence of a large number of pores on the surface;
• low impact resistance;
• generally less mechanical capacity than a forged part;
• lower density;
• noise absorption;
• possibility of using liquid immiscible alloys;
• possibility of using self-hardening alloys;
• low production cost for a series of parts.

These characteristics define the market penetration power of the technique of producing parts by powder metallurgy but this shows also its limits.

The porosity on the surface prevents the production of mechanical parts in front resist impact and / or abrasive wear due to the shortness of the period initiation of cracks compared to a forged or machined part. It's here reason why the mechanical parts obtained by powder metallurgy are traditionally not used in very abrasive or high voltage. It is here that the mechanical parts according to the present invention, more particularly WC coating by laser deposition, are a concept revolutionary for this industry sector.

• la surface de la pièce est fusionnée sur une épaisseur allant de 10 µm à 1 mm. Ceci permet la fermeture des pores en surface de la pièce et, par conséquent, l'accroissement de la résistance aux chocs;
• la faible surface couverte à un instant donné par le faisceau laser permet l'auto-trempage de la zone exposée, suite au déplacement du faisceau, par effet de puits de chaleur du volume métallique environnant;
• une très faible porosité du revêtement, moins que 1%, à cause de la fusion complète des poudres de Ni-9%Cr par le laser. Ceci n'est pas permis par les autres procédés de projection telles que la torche à plasma ou à acétylène, à cause du trop grand flux de chaleur projeté sur la pièce lorsque la température nécessaire à la fusion des poudres projetées est utilisée. La trempe de la pièce est alors détruite; et
• excellente adhérence du revêtement sur la pièce à cause de la zone de soudage.

As an indication, the deposition by laser of a coating formed at 65% of spherical WC particles taken within a Ni-9% Cr-Co matrix, allows the following improvements in the surface of the parts made by sintering metallic powders:

• the surface of the part is fused over a thickness ranging from 10 μm to 1 mm. This allows the closing of the pores on the surface of the part and, consequently, the increase in impact resistance;
• the small surface covered at a given time by the laser beam allows the self-soaking of the exposed area, following the displacement of the beam, by heat sink effect of the surrounding metallic volume;
• very low porosity of the coating, less than 1%, due to the complete melting of Ni-9% Cr powders by the laser. This is not allowed by other spraying methods such as the plasma or acetylene torch, because of the excessive heat flow projected onto the part when the temperature necessary for the fusion of the projected powders is used. The hardening of the part is then destroyed; and
• excellent adhesion of the coating to the workpiece due to the welding area.

• résistance très grande aux chocs à cause de la moins grande propension à l'initiation de fissures comparativement à un carbure à géométrie angulaire;
• limitation de l'usure par frottement à cause du coefficient de frottement plus faible des carbures sphériques comparativement aux carbures à géométrie angulaire; et
• limitation pure et simple de l'usure de la surface des pièces à cause de la dureté des carbures.

In addition, the coating obtained according to the present invention, comprising spherical carbides, has the following advantages:

• very high impact resistance due to the lower propensity for crack initiation compared to a carbide with angular geometry;
• limitation of wear by friction because of the lower coefficient of friction of spherical carbides compared to carbides with angular geometry; and
• outright limitation of wear on the surface of the parts due to the hardness of the carbides.

In addition, a Ni-9% Cr matrix, as described above, has a excellent toughness, superior to steel.

• excellente adhérence du revêtement à cause du lien métallurgique entre le revêtement et le métal de base;
• contrairement aux techniques de dépôts par projection plasma, absence de porosité et de fissures résultant en une bonne résistance aux chocs;
• épaisseur à partir de 0.5 mm jusqu'à plusieurs millimètres (rechargement de pièce possible); et
• les particules de carbure restent solides pendant le procédé de dépôt, conservant ainsi leur dureté élevée.

In summary, a sintered part comprising a coating according to the present invention has the following advantages:

• excellent adhesion of the coating due to the metallurgical bond between the coating and the base metal;
• unlike plasma spray deposition techniques, absence of porosity and cracks resulting in good impact resistance;
• thickness from 0.5 mm up to several millimeters (part reloading possible); and
• the carbide particles remain solid during the deposition process, thus retaining their high hardness.

The applications of the present invention can be found in a multitude of areas. More particularly, the debarker pellets mounted on the debarking arms can advantageously be manufactured according to the present invention as well as each of the parts mentioned above.

Claims (14)

1. A mechanical part (4) with abrasionproof surface comprising:

   a sintered metallic body (10) obtained by powder metallurgy ; and

   a cermet coating (12) covering the metallic body and having an external surface (14) constituting the abrasionproof surface, the mechanical part being characterized in that:

   said coating (12) is obtained by laser deposit by coaxially injecting in a laser beam a flux of a mixture of metallic powders and ceramic powders comprising spheroidal-shaped carbides, said mixture being intended to form said coating, which is characterized in that it is exempt of porosity, is metallurgically bound to the metallic body (10), has a thickness ranging from 10 microns to 1 mm and comprises spheroidal-shaped carbides (16) in a metallic matrix.
2. A mechanical part (4) with abrasionproof surface according to claim 1, characterized in that the cermet coating (12) comprises carbides (16) in a metallic matrix, the carbides being selected from the group consisting of tungsten carbides, titanium carbides and boron carbides.
3. A mechanical part (4) with abrasionproof surface according to claim 2, characterized in that the carbides are tungsten carbides (16).
4. A mechanical part (4) with abrasionproof surface according to any one of claims 2 and 3, characterized in that the metallic matrix (18) comprises at least one metal selected from the group consisting of nickel, chromium and cobalt.
5. A mechanical part (4) with abrasionproof surface according to any one of claims 2 to 4, characterized in that the metallic matrix (18) comprises nickel, chromium and cobalt.
6. A mechanical part (4) with abrasionproof surface according to any one of claims 2 to 5, characterized in that the metallic matrix (18) is a Ni-9%Cr-Co matrix.
7. A mechanical part (4) with abrasionproof surface according to any one of claims 2 to 6, characterized in that the coating (12) comprises 65% in weight of tungsten carbides (16).
8. A method for manufacturing a sintered mechanical part (4) with abrasionproof surface, the method being characterized in that it comprises the following steps:

   a) providing a sintered metallic part (4) obtained by powder metallurgy; and

   b) depositing by a laser process a cermet coating (12) on an external surface of said part (4); said laser process comprising the following steps:

   guiding a laser beam (22) on the external surface of the part (4), the laser beam (22) releasing a certain temperature;

   injecting in the laser beam (22) a constant flux (24) of a mixture of ceramic powders comprising spheroidal-shaped carbides and of metallic powders intended to form the cermet coating (12), the ceramic powders having a higher fusion temperature than the temperature of the laser beam and the metallic powders having a lower fusion temperature than the temperature of the laser beam so that the laser beam fuses the metallic powders of the powder mixture that is deposited on the external surface of the part (4); the powder mixture being injected in the laser beam (22) by means of a coaxial nozzle (20) traversed in its centre by the laser beam (22), the nozzle (20) allowing the arrival of the powder mixture and its injection in the laser beam (22) and

   displacing the laser beam (22) relative to the mechanical part (4) to thus sweep the external surface of the metallic body (10) and form the cermet coating (12).
9. A method according to claim 8, characterized in that:

   the laser beam (22) is fixed and the mechanical part (4) is installed on a mobile table (30) displaceable relative to said laser beam (22).
10. A method according to any one of claims 8 and 9, characterized in that the cermet coating (12) comprises tungsten carbides (16) in a metallic matrix (18).
11. A method according to any one of claims 8 to 10, characterized in that the ceramic powder is a powder of tungsten carbides and in that the metallic powder is a powder comprising at least one of the elements of the group consisting of nickel, chromium and cobalt.
12. A method according to claim 11, characterized in that the metallic powder is a Ni-9%Cr-Co powder.
13. A barking tool (4) comprising a metallic body (10) with a lower face adapted to be mounted on the extremity of a barking arm (2) and an abrasionproof working surface, the tool being characterized in that:

    the metallic body (10) is a sintered metallic body obtained by powder metallurgy; and

    the abrasionproof working surface consists of a cermet coating (12) covering the metallic body (10), the coating (12) having a thickness ranging from 10 µm to 1 mm metallurgically bound to the metallic body.
14. A barking tool (4) according to claim 13, characterized in that the cermet coating (12) is a coating obtained by laser deposit on the metallic body (10).

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